Collagen remodeling by phagocytosis is determined by collagen substrate topology and calcium-dependent interactions of gelsolin with nonmuscle myosin IIA in cell adhesions

We examine how collagen substrate topography, free intracellular calcium ion concentration ([Ca2+](i), and the association of gelsolin with nonmuscle myosin IIA (NMMIIA) at collagen adhesions are regulated to enable collagen phagocytosis. Fibroblasts plated on planar, collagen-coated substrates show minimal increase of [Ca2+](i), minimal colocalization of gelsolin and NMMIIA in focal adhesions, and minimal intracellular collagen degradation. In fibroblasts plated on collagen-coated latex beads there are large increases of [Ca2+](i), time- and Ca2+-dependent enrichment of NMMIIA and gelsolin at collagen adhesions, and abundant intracellular collagen degradation. NMMIIA knockdown retards gelsolin recruitment to adhesions and blocks collagen phagocytosis. Gelsolin exhibits tight, Ca2+-dependent binding to full-length NMMIIA. Gelsolin domains G4-G6 selectively require Ca2+ to interact with NMMIIA, which is restricted to residues 1339-1899 of NMMIIA. We conclude that cell adhesion to collagen presented on beads activates Ca2+ entry and promotes the formation of phagosomes enriched with NMMIIA and gelsolin. The Ca2+-dependent interaction of gelsolin and NMMIIA in turn enables actin remodeling and enhances collagen degradation by phagocytosis.